1. Field of the Invention
The present invention relates to a system to control the vertical movement of a drillstring within a drill rig and, more particularly, to such a system which is removably insertable into a conventional drill rig, such as one having cable draw works.
2. Setting of the Invention
In rotary drilling of wellbores, it is desirable to continue the drilling operation with a particular drill bit as long as possible to prevent unnecessary trips out of the wellbore to change drill bits; these trips for bit changes can dramatically increase the cost of the drilling operation. Several new types of drill bits have been developed which have much longer operating lives than previously developed bits, however, it has been found that these new drill bits, and especially polycrystalline diamond bits, are very sensitive to the weight-on-bit (WOB), that is, the optimum penetration rate of these new bits falls within a narrow range of WOB. The diamond cutters on these drill bits are rapidly destroyed if the WOB is too high due to either a sudden change in the formation or by a WOB addition that is in too large of an increment. When using these new drill bits, it is important to closely monitor the WOB to achieve the maximum life and efficiency of these drill bits.
Further, these new drill bits have been found to have greater penetration rates at RPM's higher than the usual drill bits and at an RPM which is higher than normally used with conventional rotary tables. Therefore, these new drill bits are often used with high RPM downhole turbines and motors. Unfortunately, these downhole turbines and motors are very sensitive to torque caused by rapid changes in WOB. For optimum performance when using a downhole turbine or motor, the WOB is preferably controlled to within a tolerance of less than about .+-.200 lbs. It has been found, however, that drill rigs which use a conventional cable draw works are not very accurate in controlling WOB because the cables have a certain amount of elasticity which can cause a surge in WOB, as well as the brake-type feed system on the cable draw works is usually human controlled and thus is not very accurate. It has been found that even with experienced drilling operators, the WOB can only be controlled consistently to within a tolerance of no more than about .+-.700 lbs, which is not acceptable in utilizing the high RPM downhole turbines and motors and/or these new drill bits.
Various devices have been developed to control the WOB; these include designing in finer tolerances in the cable draw works and gearing, as well as automatic brake-type feed systems. However, all of these devices are not accurate enough for use with the new drill bits and for use with certain high RPM downhole turbines and motors. Another type of WOB control device includes a monitor and alarm system whereby the WOB and RPM of the drillstring or the drill bit is electronically monitored, and if either of these vary outside of a preset range, then either an alarm will sound, or a microprocessor can be included to control the draw works operations and the rotary table to adjust the WOB and RPM. These WOB control devices are very expensive and have not been effective in the field due to high maintenance and, more importantly, still include the previously discussed problems inherent with a cable draw works. Another WOB control device on a drill rig is a large, long stroke hydraulic cylinder and piston assembly used in place of the cable draw works, and are called hydraulic drill rigs. These hydraulic drill rigs have not found favor in the industry and not been utilized due to their high cost and certain inherent problems with such large hydraulic systems.
Other hydraulic systems have been developed for control of WOB and have been used in offshore drilling operations, and are called "heave compensators" used to prevent the heaving motion of a drillship from affecting the WOB of the drillstring. Such hydraulic systems are disclosed within U.S. Pat. Nos. 3,653,635; 3,718,316; 3,793,835; No. Re. 29,564; and 3,871,622. All of these patents disclose heave compensators to maintain a constant WOB for use with cable draw works; however, there is no disclosure or suggestion in any of these patents of a system which utilizes a fluidic cylinder and piston assembly for precisely controlling the vertical movement of the drill string and which is easily installed and removed from a conventional drill rig having cable draw works. Further, there is no suggestion or disclosure within any of these patents of such a removable fluidic cylinder and piston assembly for controlling the vertical movement of a drill string in response to data received from a data measurement system.
A different type of heave compensator is disclosed in U.S. Pat. No. 3,905,580 and includes a modified cable draw works used with a hydraulic assembly for WOB adjustment. There is no disclosure or suggestion within this patent of a fluidic cylinder and piston assembly for precisely controlling the vertical movement of a drillstring and, which is easily installed and removed from a conventional drill rig having cable draw works. Further, there is no disclosure or suggestion within this patent of such a removable fluidic cylinder and piston assembly for controlling the vertical movement of a drill string in response to data received from a data measurement system.
The concept of controlling the movement of a drillstring in response to data received from data measurement system is disclosed in an article written by F. S. Young, Jr., Humble Oil and Refining Corporation, and presented at the SPE 43rd Annual Fall Meeting in Houston, Tex., Sept. 29-Oct. 2, 1968. However, Young does not disclose or suggest the use of a fluidic cylinder and piston assembly for precisely controlling the movement of a drillstring, nor does the Young article disclose or suggest such a system which is easily connected to and removed from a conventional drill rig having cable draw works.